1. Field of the Invention
This invention relates to an electromagnetic device which converts electrical energy into mechanical energy.
2. Prior Art
The ever growing use of electronic controls in automobiles has led to an increase in application of electric actuators. At present, solenoids are the most widely used electric actuators in automotive controls. In the past, solenoids have been used mostly to perform occasional switching functions in which the response time of the solenoid was not very important. However, the recent advances in automotive electronics have led to increased usage of solenoid actuators in performance of functions of substantial complexity, such as control and operation of a fuel injection system, in which the response of the solenoid to the control signal and the speed of its operation are critical to the overall performance of the system.
The response of a solenoid to a voltage signal is determined by two factors: the time constant of the solenoid coil and the ratio of the magnetic traction force to the moving mass. The time constant determines the time delay involved in building up the magnetic force to the required magnitude, while the force to mass ratio represents the acceleration of the moving mass. It is easier to achieve fast response in small solenoids producing small forces than in large units capable of generating substantial traction forces. Nevertheless, it is the ability of a solenoid to combine a large force capability with a very fast response that often is the most sought after property of a solenoid actuator.
Analysis of mathematical relationships between various parameters of a solenoid coil indicates that the time constant T can be approximately expressed as a function of three parameters: the traction force of a single magnetic pole F, the initial air gap length l, and the power input P. The time constant of a solenoid coil is directly proportional to the product of the traction force and the air gap length and inversely proportional to the electrical power input, EQU T=(2 F l/P
The force F and the air gap length l in the above equation are usually fixed design parameters of the solenoid. Therefore, for given values of the traction force and the air gap length, the time constant is a function of the input power only, to which it is inversely proportional. A fast response solenoid is a high energy solenoid and must have a high power to force ratio, at least during the activation period.
For a given air gap length, increase in the traction force leads to an increase in the time constant, unless the electric power input is increased in the same proportion as the force. Unfortunately, an increase in the electric power input is limited by the ability of the system to reject waste heat generated in the solenoid. Attempts to overcome this difficulty include using forced liquid cooling applied to a coil which is run at high temperature. An increase in the temperature of the coil is, of course, restricted by the ability of its materials to withstand heat. As a result, there is a limit to the amount of energy which can be safely put into a given induction coil.
In small induction coils with large surface to cross-sectional area ratios, reasonably high power to force ratios can be achieved. It is much more difficult, however, to achieve such favorable power to force ratios in large coils designed for large traction forces. One of the main reasons for this is that, if we attempt to prevent an increase in the time constant by increasing the power input at the same rate as the traction force, there is no corresponding increase in the volume and outer surface of the copper wire in which the heat is generated. Because of that the heat transfer conditions grow progressively worse and the coil overheats. As a result, large induction coils usually are restricted to smaller power to force ratios and have larger time constants than those which can be achieved in small coils.
The force to the moving mass ratio, usually, declines with increase in the force and size of the coil. This is due to the fact that the increase in force is proportional to the increase in the face area of the armature, while the moving mass is proportional to the volume of the armature which, due to a corresponding increase in its length, grows faster than the face area. This leads to smaller accelerations and, consequently, longer travel times in larger coils. Therefore, the response of a conventional solenoid becomes slower with increase in the force and size of the solenoid coil, due to concurrent increase in time constant and decrease in acceleration.
The prior art also teaches helical solenoid actuators as described in "Helenoid Actuators - A New Concept in Extremely Fast Acting Solenoids" by A. H. Seilly, Society of Automotive Engineers Technical Paper 790119, 1979. This construction uses a single magnetic core which is elongated and wound into a helical shape. The shape is generally of an E-shaped solenoid extended in a direction perpendicular to the three prong extensions of the E-shape. Such a shape is relatively difficult to fabricate and causes a certain amount of flux leakage which is then unavailable for creating armature movement. These are some of the problems this invention overcomes.